Closed-cell foam and the like expanded cellular products of conventional non-crosslinked thermoplastic polymers usually tend to very rapidly collapse when exposed to heat at a temperature level that is more than about 20.degree. C. or so above either the glass transition temperature (T.sub.g) or the crystalline melting point (T.sub.m) of the involved polymer.
It is known that lightly cross-linked thermoplastic polymers will expand to good quality, low density foams capable of resisting collapse at temperatures that are as great as 50.degree.-100.degree. C. and more higher than the collapse temperature of the corresponding, non-cross-linked polymer in the involved system. Generally, such lightly cross-linked polymer systems are provided so as to have between about 1 and about 3 actual cross-links per weight average molecule of the therein involved polymer. Copolymers of styrene with minor amounts of divinylbenzene (such as those containing on the order of about 0.04 percent by total molecule weight of interpolymerized divinylbenzene and which may conveniently be designated by the description "styrene-co-0.04%-divinylbenzene") are typical of such lightly cross-linked polymer systems. Also representative of same are analogous products of various ethylene, propylene, etc., polymers and copolymers cross-linked chemically and/or by ionizing radiation influences and the like.
Ordinarily, however, such lightly cross-linked polymer systems are incapable of fabrication into foam products by melt processing techniques if any satisfactory degree of thermocollapse resistance is desired in the expanded goods finally obtained; the reason for this being that mechanical shear almost unavoidably tends to at least materially disrupt if not destroy the involved cross-linked network structure thereby serving to both seriously degrade not only foaming behavior but to substantially, if not entirely, impair desired enhancement of thermocollapse resistance capability.
The relevant state of the art in the area of present interest is well represented and brought forth by U.S. Pat. Nos. 2,848,427; 2,848,428; 2,948,664; 2,948,665; 2,952,594; 3,098,831; 3,098,832; 3,264,272; 3,526,652, 3,717,559; 3,878,133; 3,940,517; 3,960,784; 3,986,991; 4,169,921 and 4,234,531 (the respective subject matter and disclosures of each and every one of which are herein contemplated and incorporated by reference).
Nonetheless, nothing known prior hereto appears to realistically concern itself with suitable polymeric compositions that are well adapted for conversion by normal melt processing procedures into fine quality, closed-cell, thermocollapse resistant foams in the way so indigenously advantageous as in the present contribution to the art.